The mandatory formation of a trimolecular complex consisting of processed antigen, MHC class II molecule and T-cell receptor for activation of helper T cells has become one of the paradigms of modern immunology. This application proposes to examine the role of one of the components of this complex, the MHC class II molecule (Ia antigen), in this process. These studies will exploit structure-function analysis of the Ia antigens as well as make extensive use of anti-class II monoclonal antibodies and chemical crosslinkers. The overall goal of defining the functional sites of the I-Ak molecule that are involved in the interaction of the class II molecule with peptides (representing processed antigen) and with the T-cell receptor will be accomplished through the completion of four specific aims. First, the epitopes detected by the monoclonal antibodies will be mapped using chemical crosslinking and protein microsequence analysis to identify those portions of the I-Ak molecule in contact with the monoclonal antibodies or in close proximity to its binding site. Second, anti-I- Ak monoclonal antibodies and mutant I-Ak molecules will be employed to locate and to define the biochemical properties of the peptide-binding and T-cell receptor-contacting sites of the I- Ak molecule. Third, synthetic peptides corresponding to the hypervariable region sequences of the A alpha and A beta chains will be used to probe the antigen/I-A/T cell receptor interactions. Studies under this specific aim will determine the effect these peptides have no antigen binding and on antigen presentation by I- A molecules and will attempt to establish the mechanism(s) by which the hypervariable region peptides act. Fourth, chemical crosslinkers and protein microsequencing will be employed to locate the portions of the I-A molecule that make contact with the bound peptide antigen. The proposed studies derive medical significance from the fact that the human class II molecules, HLA-DR, -DP and -DQ show clear associations with certain disease states in man. Recent studies have suggested that allelic variation in hypervariable regions homologous to those studied in the present application may explain these associations. Clearly, the understanding of the structure-function relationships of the class II molecules should facilitate the design of rational treatments and/or preventatives for these HLA-associated diseases.